Fix auto preload <img> issue for renderToString

[mimosapudical]

Fix for React 19 renderToString() preload injection bug (#34219)

Changes

• Added disableAutoPreload flag to createRenderState
• Attached renderState to request object
• Modified flushCompletedQueues to check flag before injecting preload links

Summary

This PR fixes the unexpected behavior in React 19 where renderToString(<img />) automatically injects <link rel="preload"> for every image. React 18 did not do this. By adding a disableAutoPreload flag in createRenderState and checking it in flushCompletedQueues, developers can now opt out of automatic preload injection when needed, restoring the expected SSR output.

How did you test this change?

• Created a minimal test case with renderToString(<img src="my/image.png" />)
• Verified that with disableAutoPreload = true, the output is:

<img src="my/image.png"/>

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[sebmarkbage]

I'm not sure if we want to do this at all given that this is a feature of suspensey images. The way to opt out of it is to either make the image loading="lazy" or add an onLoad event listener.

But regardless, the fix is in the wrong place since it's way too broad and violates the contract by reaching into the type that varies by context. It would go into an override in:

https://github.com/facebook/react/blob/main/packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js

The more interesting one might be the markup renderer since that's not expected to be rendered in the same way it might not need a preload in the same way. E.g. you don't preload from e-mails. That's also likely to be the replacement when renderToString gets deprecated anyway.

[alexanderson1993]

I'm not sure if we want to do this at all given that this is a feature of suspensey images. The way to opt out of it is to either make the image loading="lazy" or add an onLoad event listener.

This behavior doesn't seem to be documented on react.dev. It seems like this would be a reasonable section for such a doc to go: https://react.dev/reference/react-dom/components

[mimosapudical]

I'm not sure if we want to do this at all given that this is a feature of suspensey images. The way to opt out of it is to either make the image loading="lazy" or add an onLoad event listener.

But regardless, the fix is in the wrong place since it's way too broad and violates the contract by reaching into the type that varies by context. It would go into an override in:

https://github.com/facebook/react/blob/main/packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js

The more interesting one might be the markup renderer since that's not expected to be rendered in the same way it might not need a preload in the same way. E.g. you don't preload from e-mails. That's also likely to be the replacement when renderToString gets deprecated anyway.

The attempted fix adds an empty preloadInstance override in ReactFizzConfigDOMLegacy.js, but it does not pass.

Testing: On Windows, the Jest test only passes if the else if block in pushImg within ReactFizzConfigDOM.js is removed, showing that renderToString doesn’t fully pick up the ...DOMLegacy.js config.

[kurtextrem]

@sebmarkbage I appreciate that React tries to be smart here, but adding preloads by default is maybe a bit too smart. A sensible suggestion seems to this one: #34217 (comment). As an explicit loading="eager" is likely a great developer intent something is important (and thus might be smart to be preloaded). Preloading everything that has no explicit intent seems an easy pitfall for performance.